CH. XIV] STARCH. 291 



Hot dilute acid (1 p.c.) is frequently used, being much 

 more convenient than diastase, but there is reason to 

 believe that this method under some circumstances yields 

 results seriously too high, as the acid is not without in- 

 fluence on the cellulose (compare E. Schulze, Ber. d. d. Chem. 

 Ges. xxiv. (1891), p. 2277, but also Winterstein, loc. cit.). 



With both reagents the starch is first converted into 

 soluble products (dextrin and maltose, or dextrin, maltose 

 and glucose), and the products are then further hydrolysed 

 to glucose by continued action of stronger acid. The 

 latter stage of the process is liable to error in both 

 directions, the hydrolysis may not be complete or the 

 glucose may be partly destroyed by ' reversion.' To obtain 

 good results it is necessary therefore to adhere strictly to 

 the instructions given for any process. 



Heating for half-an-hour with 1 p.c. sulphuric acid in a 

 sealed tube at 120 C. (pressure about 2 atmospheres) gives 

 good results with pure starch. 



A full account of Allihn's and other methods for 

 estimating starch is given in Tollens' work. 



Starch. 



The whole of the soluble products obtained by limited 

 hydrolysis of starch will be contained in the dilute acid 

 extract (Extract IV.), and there will generally be little else 

 in this extract. Any proteids etc., which it may contain 

 may be neglected. 



Qualitative. 



The original material is tested for solid starch either 

 microchemically or by Sachs' iodine method. 



192 



